Original Paper Brain Behav Evol 2021;96:1–12 A Farewell to the Encephalization Quotient: A New Brain Size Measure for Comparative Primate Cognition Carel P. van Schaik a, b Zegni Triki c, d Redouan Bshary c Sandra A. Heldstab a a Department of Anthropology and Anthropological Museum, University of Zurich, Zurich, Switzerland; b Department of Evolutionary Biology and Environmental Science, University of Zurich, Zurich, Switzerland; c Behavioral Ecology Laboratory, Faculty of Science, University of Neuchâtel, Neuchâtel, Switzerland; d Institute of Zoology, Stockholm University, Stockholm, Sweden Received: February 17, 2021 Accepted: May 2, 2021 Published online: July 9, 2021 Correspondence to: Carel P. van Schaik, vschaik @aim.uzh.ch © 2021 The Author(s) Published by S. Karger AG, Basel karger@karger.com www.karger.com/bbe DOI: 10.1159/000517013 Keywords Encephalization quotient · Cognitive equivalence · Intelligence · Mammals · Hominins Abstract Both absolute and relative brain sizes vary greatly among and within the major vertebrate lineages. Scientists have long debated how larger brains in primates and hominins translate into greater cognitive performance, and in particu- lar how to control for the relationship between the noncog- nitive functions of the brain and body size. One solution to this problem is to establish the slope of cognitive equiva- lence, i.e., the line connecting organisms with an identical bauplan but different body sizes. The original approach to estimate this slope through intraspecific regressions was abandoned after it became clear that it generated slopes that were too low by an unknown margin due to estimation error. Here, we revisit this method. We control for the error problem by focusing on highly dimorphic primate species with large sample sizes and fitting a line through the mean values for adult females and males. We obtain the best esti- mate for the slope of circa 0.27, a value much lower than those constructed using all mammal species and close to the value expected based on the genetic correlation between brain size and body size. We also find that the estimate of cognitive brain size based on cognitive equivalence fits em- pirical cognitive studies better than the encephalization quotient, which should therefore be avoided in future stud- ies on primates and presumably mammals and birds in gen- eral. The use of residuals from the line of cognitive equiva- lence may change conclusions concerning the cognitive abilities of extant and extinct primate species, including hominins. © 2021 The Author(s) Published by S. Karger AG, Basel Introduction Although recent ecological approaches to comparative cognition have focused on linking performance in spe- cific cognitive tasks to specific brain regions [e.g., Healy and Krebs, 1996], traditionally comparative cognition has relied on a presumed link between some summary mea- sure of cognitive performance and total brain size [Jeri- son, 1973]. Scholars have therefore long been searching for a neuroanatomical measure of overall cognitive abil- ity, both to compare living species and to estimate the Tis is an Open Access article licensed under the Creative Commons Attribution-NonCommercial-4.0 International License (CC BY-NC) (http://www.karger.com/Services/OpenAccessLicense), applicable to the online version of the article only. Usage and distribution for com- mercial purposes requires written permission.